Fix TypeParam.rename to substitute variables in bounds and default types - #3068
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The substitution in TypeParam.rename was built from the new names to themselves, making it an identity substitution. Type variables in upper bounds, lower bounds, and default types that reference other type parameters were left with the old names. One visible consequence: `validate_type_params` raised a false GenericParameterMismatchError for declarations like `module M[A, B < _Foo[A]]` and `module M[X, Y < _Foo[X]]`, which are compatible modulo renaming. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01TBY8ct4HpkVkZNPdsNHEDE
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TypeParam.renamebuilt its substitution from the new names to themselves, making it an identity substitution, so type variables in upper/lower bounds and default types that reference other type parameters kept the old names. One visible consequence:validate_type_paramsraised a falseGenericParameterMismatchErrorfor compatible declarations likemodule M[A, B < _Foo[A]]andmodule M[X, Y < _Foo[X]]. The substitution now maps the old parameter names to the new variables.